In certain pathologies, the spinal canal extending through a patient's vertebrae is or becomes too narrow and constricts the spinal cord extending therethrough. The narrowing may be congenital, potentially affecting patients at any age. Narrowing can also be attributable to other causes, such as age, injury or removal of a spinal disc.
A condition associated with aging, for instance, is spondylolsis, in which intervertebral discs lose water and become less dense. These degenerative changes near the disc can cause an overgrowth of the bone, producing bony spurs called, “osteophytes” that can compress the spinal cord. The constriction of the spinal cord in the cervical spine, for example, often produces pain, weakness, or loss of feeling in extremities. Other causes for narrowing of the spinal canal include disc shrinkage, which causes the disc space to narrow and the annulus to bulge and mushroom out, resulting in pressure on the spinal cord. Degenerative arthritis of facet joints can cause joints to enlarge, or the vertebrae to slip with respect to each other, also compressing the spinal cord. Instability between vertebrae, such as caused by stretched and thickened ligaments can also produce pressure on the spinal cord and nerve roots.
Myelopathy, or malfunction of the spinal cord, occurs due to its compression. The rubbing of the spine against the cord can also contribute to this condition, and the spinal cord compression can ultimately compromise the blood vessels feeding the spinal core, further aggravating the myelopathy.
Traditional procedures for decompressing the spinal cord include a laminectomy, in which the lamina and spinal processes are removed to expose the dura covering the spinal cord. Laminectomies, however, can lead to instability and subsequent spinal deformity. Another known procedure is a laminoplasty, in which the lamina is lifted off the dura, but not completely removed. Typically, one side of the lamina is cut, while a partial cut is made on the other side to hinge the lamina away from the spinal cord to increase the size of the spinal canal. A strut of bone can be placed in the open portion within the lamina and the facet to help hold the open position of the lamina. Laminoplasties preserve more of the bone, muscle, and ligaments, but current techniques and devices are cumbersome and require exceptional technical skills, particularly for performing minimally invasive techniques.
Accordingly, improved methods and devices for expanding the spinal canal are needed, and in particular, methods and devices that can be used in minimally-invasive surgery.